Did Washington Monument sink or tilt from quake?

A smaller obelisk in a well 12-feet deep which was used as a geodetic marker and referred to as "the mini monument", is seen at the base of the Washington Monument in Washington, Tuesday, March 13, 2012. Government surveyors are collecting data around the Washington Monument and other sites on the National Mall that will reveal whether it has sunk or tilted since last yearÍs earthquake.

Charles Dharapak, Associated Press

Summary

The Washington Monument cracked and crumbled when a 5.8-magnitude earthquake shook the nation's capital last year. But did it sink or tilt?

WASHINGTON — The Washington Monument cracked and crumbled when a 5.8-magnitude earthquake shook the nation's capital last year. But did it sink or tilt?

A team of government surveyors is trying to find out.

On Tuesday, surveyors were on the grounds of the 555-foot-tall obelisk, taking measurements from several long-established points in the ground known as bench marks where survey work has been done in the past.

They're not expecting to find any major changes — perhaps fractions of an inch. But the findings could affect plans for repairing the monument, which is expected to remain closed to visitors until next year.

"Obviously the event was not so significant that we see big cracks in the ground," said David Doyle, chief geodetic surveyor with the National Geodetic Survey, which is conducting the survey. "Whatever changes have occurred here would be much, much more subtle."

The monument sits about 15 to 20 feet above sea level and has sunk about 2 inches into the ground since it was completed in 1884. It is on land that was once underwater. Most of the National Mall was created with soil dredged from the Potomac River. That's one reason why the structures on the mall are prone to settling in the ground, leading to problems that required major renovations of the Jefferson Memorial plaza and the Reflecting Pool.

Several large cracks and dozens of smaller ones formed in the top portion of the monument during the earthquake on Aug. 23, and chunks of stone were shaken loose on the exterior and interior of the structure. Although it remains structurally sound, repairs are expected to cost at least $15 million, and the monument could remain closed until August 2013.

The last survey of the monument was conducted in 2009, so any settling could likely be blamed on the earthquake, although it's impossible to prove what caused it.

Although the current survey, which started last week, could determine whether the monument has tilted, that likely won't be known until examiners place a global-positioning device on the top of the structure, which last happened in 1999. That won't be done until repairs are under way.

The NGS is a division of the National Oceanic & Atmospheric Administration, an agency created in 1807 by Thomas Jefferson that's responsible for the nation's altitude, longitude and latitude measurements.

Among the points where the height of the monument has been measured in the past is a structure beneath a manhole cover near the base that's known colloquially as the "mini-monument." It was put in place when the monument was completed.

The surveyors also use two steel rods drilled into the base of the monument in 1984 that can provide especially accurate measurements.